The morphology, particle size distribution and surface area of the fly ash are covered in this section.
2.4.1 Fly Ash Morphology
The morphology of fly ash particle describes the size, shape, or structure and surface properties of the particle and is determined by combustion temperature and cooling rate in the power plant (Kutchko and Kim, 2006). The study of the fly ash particle
27
morphology is important in understanding the physical properties and leaching behaviour in terms of toxicology and environmental studies of the fly ash (Singh, 2005).
Fisher et al., (1978) used light microscopy to analyse fly ash particle morphology which they classified into eleven categories based on opacity, shape and type of inclusions.
1) Amorphous, non-opaque 2) Amorphous, opaque
3) Amorphous, mixed opaque and non-opaque 4) Rounded, vesicular, non-opaque
5) Rounded, vesicular, mixed opaque and non-opaque 6) Angular, lacy, opaque
7) Cenosphere (hollow sphere), non-opaque
8) Plerosphere (sphere filled with other spheres), non-opaque 9) Nonopaque, solid sphere
10) Opaque, sphere
11) Sphere with either surface or internal crystals, non-opaque
These also stated that the relative abundances of the eleven morphological particle categories within each size regime seem to rely on particle size. The majority of the particles in the finer fractions are spherical, glassy and mostly non-opaque showing complete melting of the silicate minerals in the coal particle. The minor opaque spheres are usually iron oxide particles like magnetite. Kutchko and Kim, (2006) also reported that the fly ash samples were comprised of over 50 % amorphous alumino- silicate spheres and a lower quantity of iron-rich spheres. The finest fraction is made up of 87 % non-opaque solid spheres and 7.9 % cenospheres whereas the coarsest fraction comprises of 26 % non-opaque solid spheres and 41 % cenospheres. Various morphological studies show that fly ash consists of a range of spherical and irregularly shaped particles of different sizes ranging from <1µm to >200 µm formed from the various physical and chemical reactions that occur during the coal combustion process. The combustion heat causes the inorganic minerals in coal to
28
fluidise or volatilise or to react with oxygen; which during cooling, may form crystalline solids, spherical amorphous particles or condense as coating on particles. Agglomerated particles are produced due to high temperature sintering reactions; spherical amorphous particles are formed from the fast cooling in the post-combustion zone; hollow cenospheres result from the expansion of trapped volatile matter that can cause the particle to expand while plerospheres are hollow spheres incorporating fly ash spheres (Fisher et al., 1978; Seames 2003; Cho et al., 2005; Kutchko and Kim, 2006; Saikia et al., 2006).
The surfaces of particles are generally smooth in fly ash that has not weathered whereas in weathered fly ash the particle surfaces have features such as encrustations, corrosions and etching which may have resulted from leaching or formation of new mineral phases as a result of weathering (Praharaj et al., 2002; Yeheyis et al., 2009).
Fly ash colour is highly influenced by the mineral phase and is mostly determined by two components. The remaining unburned carbon resulting from incomplete combustion of coal is responsible for the grey and black colour; while iron oxide with its characteristics colour depending on oxidation state of iron is another important component in determining the colour of fly ash. Trivalent iron (i.e. Fe³⁺) is brown, red, or yellow, while bivalent iron (i.e. Fe2+) is grey or grey with a bluish tinge. Magnetite which contains both Fe3+ and Fe2+ is black and can be brown when finely dispersed (Raclavska et al., 2009).
2.4.2 Particle Size Distribution and Surface Area
Fly ash consists of a range of spherical and irregular shaped particles of different sizes; the alumino-silicate iron-rich spheres spherical particles are usually small in size (between 0.1 µm and 100 µm) whereas the irregular shaped particles which consist mostly of unburned carbon are larger (<1µm - >200 µm) (Styszko- Grochowiak et al., 2004; Potgieter-Vermaak et al., 2005; Cho et al., 2005). A number of related factors such as the size distribution of the coal particle and the accessory minerals, combustion conditions and the particulate emission control devices determine the particle size distribution of fly ash (Fisher, 1983). The size and
29
distribution of these particles is the most important characteristic determining its reactivity. The smaller particles have greater specific surface area making a larger area susceptible to hydrolysis. The particle size distribution is important during interaction of fly ash with different solutions because it affects the mobilisation of any trace element on the surface (Mattigod et al., 1990; Iyer, 2002; Jankowski et al., 2006). The particle size distribution also plays a role in the concentrations of some elements in fly ash. Fisher et al.,(1977) reported that the concentrations of some elements are dependent on particle size while some are independent of particle size. The highest size dependence is exhibited by the most volatile elements (Cd, Zn, Se, As, Sb, W, Mo, Ga, Pb and V) or their oxides while the least volatile elements such as (examples of the elements) do not exhibit significant particle size dependence. Extension of vapour-phase condensation to the sub-micrometre regime and homogeneous nucleation were attributed to the particle size dependence of the trace elements in fly ash.
The size of the particle seems to have an effect on density variations in the fly ash. There is an inverse variation of apparent density with particle sizes which may be as a result of the higher relative abundance of cenosphere particles and lower relative abundance of solid, non-opaque spheres in coarse fractions (Fisher et al., 1978). The particle size of a material can be important in understanding the physical and chemical properties of fly ash and plays a very important role in the utilization and disposal of fly ash. If fly ash is to be considered as a partial replacement for cement a low bulk density is required to make it ideal as a lightweight building material (Ural, 2005). The particle-size distribution of fly ashes is also a vital factor for their use as pozzolans. ASTM 618C requires that 34% of the ash must remain on a 45 mm sieve on wet sieving.